The invention relates to hydrostatic spray guns in general, and more particularly to improvements in nozzle assemblies for hydrostatic spray guns. Still more particularly, the invention relates to improvements in nozzle assemblies of the type wherein an adapter which is connectable to a barrel or to another support is made of an electrically insulating material and carries one or more high-voltage electrodes serving to ionize air which is used to atomize the material to be sprayed as such material issues from the orifice of the nozzle in the nozzle assembly.
German Auslegeschrift No. 24 46 022 of Vohringer discloses a nozzle assembly wherein the adapter has a flat front surface with a centrally located bore constituting the material discharging orifice. The orifice is surrounded by an annulus of holes which discharge streams of air serving to atomize the material which issues from the orifice. Two ported horn-like projections of the adapter extend forwardly beyond the front surface at opposite sides of the orifice and are located outwardly of the annulus of holes; their ports discharge streams of auxiliary air which is intended to promote the atomizing action of air issuing from the aforementioned holes or to shape the spray of material which issues from the orifice. High-voltage electrodes extend from the projections in close or immediate proximity to the respective ports in order to ionize the air streams. The tips of the electrodes extend from the projections through distances in the range of several millimeters, and they are located close to the periphery of the nozzle assembly. The function of such electrodes is to ionize air which issues from the ports of the respective projections as well as to ionize air which is drawn toward the spray by streams of air issuing from the ports and/or from the holes surrounding the orifice. When the ionized air contacts the spray, it transfers its charge to the droplets of the material to be sprayed so that the droplets can readily follow the paths which are determined by the lines of the electrostatic field. The locus of impingement of ionized air upon the material of the spray is determined by the primary function of air, namely by its atomizing or spray forming action.
In accordance with another earlier proposal which is disclosed in U.S. Pat. No. 3,764,068 to Lacchia, the orifice which serves to discharge the material to be sprayed is surrounded by six equidistant hollow needle-like electrodes which project well beyond the front side of the adapter. The orifice for the material to be sprayed is a ring-shaped opening which is surrounded by a second ring-shaped ofifice for atomizing air. The electrodes are placed close to the periphery of the adapter and their passages are designed to deliver streams of liquid (with or without air). Such liquid is ionized and is caused to contact the material issuing from the inner ring-shaped orifice. A drawback of the patented nozzle assembly, as well as of that which is disclosed by Vohringer, is that the electrodes are not adequately shielded because they project well beyond the adjacent portions of the adapter. Moreover, the electrodes are close to the periphery of the nozzle assembly so that the likelihood of sparking is very pronounced if the nozzle assembly is moved close to a grounded object.
Another presently known nozzle assembly is disclosed in German Utility Model No. 77 17 280 of S.K.M. This nozzle assembly employs two horns which are disposed at opposite sides of the orifice for the material to be sprayed and project well beyond the front surface of the adapter. The horns carry electrodes which protrude toward the path for the spray, and additional electrodes are installed in the adapter to project from its front surface. The electrodes which are installed in the horns are likely to come into contact with the object or objects to be coated.
German Offenlegungsschrift No. 26 15 360 of Buschor et al. discloses a nozzle assembly wherein the tips of the electrodes project well beyond the front side of the adapter. The latter can be provided with a ring-shaped shield which surrounds the electrodes, or the electrodes are disposed in an annular recess which is surrounded by a nut serving to couple the adapter to the barrel of a spray gun. Alternatively, the recess can be formed directly in the front side of the adapter. A drawback of the proposal of Buschor et al. is that the electrodes are exposed and are likely to be damaged or that the electrodes are overly concealed so that they cannot ensure adequate ionization of secondary air.
Further conventional nozzle assemblies for use in spray guns are disclosed in U.S. Pat. No. 3,583,632 to Shaffer et al., in U.S. Pat. No. 4,055,300 to Binoche, in U.S. Pat. No. 4,258,885 to Legeza, and in commonly owned U.S. Pat. No. 4,713,257 to Luttermoller.